1. Field of the Invention
This invention relates to a focusing method, and more particularly, to a focusing method of layer jumping for an optical storage device to retrieve data from a multi-layer optical disc.
2. Description of Related Art
As the need of storage medium with high capacity greatly increases, optical storage devices now play an essential role. It has been a critical issue to reduce the data access time when reading data from the optical disc.
In a conventional optical storage drive, a disc is disposed on a spindle motor, which rotates the disc. The disc has multiple tracks for storing digital information, which is read by an optical pickup head. The optical pickup head is disposed on a sled. A sled driving motor can drive the sled, whereby the pickup head can reach the desired position for reading information of the disc.
The reading process of an optical drive includes the following steps. First, after the optical drive is initiated, the spindle motor is driven to rotate the optical disc. The optical pickup head is then moved to the inner ring of the optical disc, and the disc is rotated above the optical pickup head. Next, a focus servo of the optical drive is driven, and a laser beam is emitted and focused to form a laser spot on the optical disc. Then, a tracking servo of the optical drive is driven to move a lens of the optical pickup head, so as to enable the laser spot to trace the target data track. After that, a track number is read out to identify the present location of the optical pickup head. Next, a long seek operation is performed, that is, the seeking servo of the optical drive is activated to move the optical pickup head from the present track to the vicinity of the target track. Then, the tracking operation is performed and the optical pickup head reads its present track number for knowing the distance from the target track. Then, a short seek operation is performed, that is, the lens is moved with fine adjustment to move the laser spot to the target track. Then, the tracking operation is activated and data of the optical disc is read out.
FIG. 1 schematically illustrates the dual actuator system used in a seeking operation. As FIG. 1 shows, the dual actuator system is composed of a sled actuator 102 and a fine actuator 104. A lens 106 of the optical pickup head (not shown in the figure) is mounted on the fine actuator 104. During the seeking operation, the sled actuator 102 moves the fine actuator 104 back and forth on the actuator track 108, so as to let the lens 106 remain at a linear region of the fine actuator 104. A proper control on the sled actuator 102 and the fine actuator 104 is necessary to assure that the laser spot can be precisely focused onto the optical disc 114, which is affixed on the damper 110 and driven by the spindle motor 112. The sled actuator 102 usually is a sled motor, and the fine actuator 104 usually is a voice coil motor (VCM). The lens 106 is coupled to the sled through a spring coil of the VCM.
A focusing operation is employed for monitoring the distance between the optical disc 114 and the lens 106 in order to keep the focus point. Taking the astigmatism as an example, an optical pickup head includes a quartered beam sensor. The radiant in the quartered beam sensor will show a different shape when the laser beam is on a different location of optical disc. Now referring to FIG. 2A that illustrates the beam sensors in the optical pickup head, the quartered beam sensor consists of 4 beam sensors, 210, 220, 230, and 240, which are for sensing beam signals reflected from an optical disc. The intensity of the reflection received by the beam sensor 210 is defined as A. The intensity of the reflection received by the beam sensor 220 is defined as B. The intensity of the reflection received by the beam sensor 230 is defined as C. The intensity of the reflection received by the beam sensor 240 is defined as D. “Focus Error” (FE) signal is defined as FE=(B+D)−(A+C). In the case of correct focusing, the 4 intensities of the reflections from the 4 beam sensors are the same. Therefore, the FE signal is 0. When the optical disc is too close to the lens, the signals A and C, are stronger, and the signals B and D are weaker. Therefore, the FE signal is positive. On the other hand, if the disc is too far from the lens, the FE signal is negative. Therefore, the focus operation can be performed according to the FE signal. Please refer to FIG. 2B, which illustrates the waveform of the FE signal. As the figure shows, when the focus point is approaching the optical disc, the FE signal is changed from plus to 0, and then to minus. It is like a sine wave signal. The area between the peak value and bottom value of the wave can be regarded as a linear area. Within the linear area, the intensity of the FE signal and the distance between the focus and the data layer (not illustrated) are linear. Therefore, the present focus point can be known from the intensity of the FE signal within the linear area. When the FE signal is 0, it means correct focusing is occurring. Therefore, when focusing, via the combination of acceleration and brake, it is impossible to get the correct focus point by one focus operation. So focus compensation is needed. But the prerequisite for focusing compensation is that the present focus is within the linear area (because the location of focus is linear to the FE signal at this time.). If the focus is not within linear area, it is impossible to have successful focusing even by the focus compensation.
At this moment, there are many formats in the Digital Versatile Disc (DVD), such as single side single layer, single side double layer, double side single layer, and double side double layer. Therefore, a layer-jump (layer jumping) mechanism is needed when data is read from a multiple-layer optical disc. Next, reference is made to FIG. 3A, which illustrates the focus operation in a multiple-layer DVD. Taking this figure as an example, an optical disc 114 is a DVD, which includes multiple data layers (a data layer 310, and a data layer 320). And as shown in FIG. 3A, a lens 106 is focused on the data layer 310. Because the focus of the lens 106 is constant, when the optical drive tries to read data from the data layer 320, the lens 106 will be moved forward to let the focus set on the data layer 320 in order to retrieve data of the data layer 320. At this moment, the lens is closer to DVD 114, as FIG. 3B shows. In the same way, when the optical drive needs to read data of the data layer 310, another layer-jump procedure is needed to let the focus set on the data layer 310.
In layer jumping, the correctness of the focus operation depends on the value of the FE signal. Please refer to FIG. 3C, which shows an error on focusing. Take this figure as an example. Suppose when a layer jumping is performed, there is a noise and therefore the lens is not set on the linear area. Even by focus compensation, the lens cannot focus on the data layer 320. The focus between the data layer 310 and the data layer 320 means a focusing failure. Another error that can happen is that during layer jumping, due to the influence of noise, the lens may be too close to the optical disc. It makes the focus over the data layer 320, which causes another focusing failure as FIG. 3D shows. The important thing is that irrespective of whether the noise makes the focus point between two data layers, or over the data layer 320 (as showed in FIG. 3D) or under the data layer 310, the FE signal is 0. Because of such a layer jump failure, the location of the lens cannot be known. Focusing must be performed again, which means spending more time and reducing the rate of data retrieving.